Printing has many problems. The print quality phenomenon known as flow streaks exists. Flow streaks can be produced by image forming apparatus such as printers (e.g. xerographic printers), photocopiers and fax machines. Flow streaks can be seen on a printed substrate, such as a piece of paper, as lines in the direction of travel of the substrate through the image forming apparatus. Typically these lines are not straight but look more like flow lines of a fluid, and are generally fainter than the printed matter on the substrate. In the more severe cases the flow lines can be seen quite frequently at various levels of intensity and appear as different grey levels on the printed substrate.
No real solutions have been proposed to this problem but attempts have been made to reduce the severity of the flow streaks by changing the working conditions of the image forming apparatus. These include increasing the ink density, increasing the flow of ink, keeping the level of ink in an ink reservoir above a certain point, and also modifying the return of the ink to the reservoir to reduce the formation of bubbles.